1. Field of the Invention
The present invention relates to a semiconductor thin film formed on a substrate having an insulating surface and a semiconductor device formed of a TFT having an active layer of the thin film. Particularly, the invention relates to a structure in a case where a material containing silicon as its main ingredient is used as a semiconductor thin film.
Incidentally, in the present specification, the term “semiconductor device” indicates any devices functioning by using a semiconductor, and the following are included in the category of the semiconductor device.
(1) Single device such as a thin film transistor (TFT).
(2) Semiconductor circuit using the single device of (1).
(3) Electro-optical device formed using (1) or (2).
(4) Electronic device (electronic equipment) including (2) or (3).
2. Description of the Related Art
In recent years, attention has been paid to a technique for constructing a thin film transistor (hereinafter referred to as a “TFT”) by using a semiconductor thin film (its thickness is several tens to several hundreds nm) formed on a substrate having an insulating surface. The thin film transistor is widely used for an electronic device such as an IC or an electro-optical device, and particularly as a switching element of an image display device, its development has been hastened.
For example, in a liquid crystal display device, an attempt to apply the TFT to any electric circuits, such as a pixel matrix circuit for controlling each of pixel regions arranged in matrix form, a driver circuit for controlling the pixel matrix circuit, and a logic circuit (a processor circuit, a memory circuit, etc.) for processing a data signal from the outside, has been made.
Under the present circumstances, although a TFT using a noncrystalline silicon film (amorphous silicon film) as an active layer has been put to practical use, a TFT using a crystalline silicon film (typically, a polysilicon film, a polycrystalline silicon film, etc.) is necessary for an electric circuit expected to have further high speed operating performance, such as a driver circuit and a logic circuit.
For example, as a method of forming a crystalline silicon film on a glass substrate, techniques disclosed in Japanese Patent Laid-Open Application No. Hei. 7-130652 and No. Hei. 8-78329 by the present applicant are well known. The techniques disclosed in these publications use a catalytic element for promoting crystallization of an amorphous silicon film, so that formation of a crystalline silicon film superior in crystallinity is made possible by a heat treatment at 500 to 600° C. for about 4 hours.
Particularly, the technique disclosed in Japanese Patent Laid-Open Application No. Hei. 8-78329 is such that crystal growth almost parallel to a substrate surface is made by applying the above techniques, and the present inventor et al. refer to a formed crystallized region especially as a side growth region (or a lateral grow region).
However, a TFT has a defect that fluctuation in electric characteristics (characteristic fluctuation) is large as compared with a MOSFET formed on a silicon wafer. Thus, it is difficult to reproduce the same characteristics even if TFTs have the same structure, which has made it difficult to form a circuit with the TFTs.
A technique for forming a high performance TFT with less characteristic fluctuation becomes necessary for realization of a system-on-panel at which the present applicant is aiming. That is, in order to realize the system-on-panel, it is necessary to use a TFT in which not only an operating speed is high (electric field mobility is large) but also fluctuation in electric characteristics representing a TFT, such as a threshold voltage and a subthreshold coefficient, is suppressed.